The invention relates to photoresist formulations for forming relief structures of highly heat-resistant polyimide polymers.
Such photoresist formulations are sought-after materials which are variously used in the preparation of photopolymerized coatings and in the photolithographic production of photopolymerized relief structures. The main uses in this area are the manufacture of microelectronic and optoelectronic components and circuits, where such materials find utility as photoresists in the production of circuit structures or therein as particular heat- and chemical-resistant protective or passivating coats, as insulation coats, dielectrics or, in the case of liquid crystal display cells, as orientation coats. Further uses are as photoresist, as etching and electroplating resist and as soldering resist in the manufacture of printed circuits, printing plates and other reprographic materials.
German Pat. Nos. 2,380,830 and 2,437,348 describe processes for preparing coatings and relief structures wherein photoresist formulations based on polyamide ester prepolymers carrying photopolymerizable radicals are imagewise exposed, unexposed portions are removed with developer and thereupon the resulting images are heat-treated, the prepolymer forming the relief structure being converted into a highly heat-resistant polyimide polymer.
However, these original systems had only moderate sensitivity, so that adequate photocrosslinking and the provision of a degree of resolution and relief structure edge steepness as required for microelectronics required minimum exposure times of about 3 minutes with conventional sources of radiation. In the case of coating thicknesses of up to about 5 .mu.m, for which range it is possible to assume an approximately linear relationship between coating thickness and required minimum exposure energy, this equates to minimum exposure energies of 500-1,000 mJ/cm.sup.2..mu.m. In the meantime, these photoresist systems have undergone various developments and improvements, in particular with regard to enhancing the sensitivity, to use shorter exposure times to obtain high economy in use.
For instance, the compositions have added to them sensitivity-enhancing additives such as, for example, photoinitiators and photosensitizers and radiation-reactive copolymerizable unsaturated compounds, if desired in combination with one another.
For instance, in German Pat. No. 2,919,841 enhanced sensitivity is obtained by addition of photoinitiators of the type of the N-axidosulphonylarylmaleimides. According to European Offenlegungsschrift No. 47,184 this is likewise possible through the addition of bisimidazole photo-initiators. Minimum exposure times around 60 seconds (corresponding to a minimum exposure energy of about 200-300 mJ/cm.sup.2..mu.m) are required to obtain sharp-edged relief structures having a resolution of about 3 .mu.m. U.S. Pat. No. 4,329,419 proposes adding to photoresist formulations based on photopolymerizable polyamide aster prepolymers a polyfunctional acrylate compound, an aromatic bisimidazole photoinitiator and a leuco dye. It emerges from said patent specification that the best obtainable minimum exposure energies are 40-120 mJ/cm.sup.2..mu.m. European Patent Application No. 0,119,162 describes photoresist materials which, in addition to photopolymerizable polyamide ester prepolymers, contain metallocenes as photoinitiators and can, if desired, also contain coploymerizable acrylate and allyl compounds. Here the best minimum exposure energy values are 30-50 mJ/cm.sup.2..mu.m. In U.S. Ser. No. 674,636 of November 26, 1984, (now abandoned) corresponding to German Patent Application No. 3,342,851, a reduction in the minimum exposure energy of polyamide ester photoresist formulations to values of 25-40 mJ/cm.sup.2..mu.m is obtained by the addition of radiation-reactive coploymerizable vinyl or allyl compounds and of photoinitiators of the type of the N-azidosuplhonylarylmaleimides.
Nonetheless, the sensitivity of such photoresist systems still does not meet the higher present-day demands for an efficient and economic production of microelectronic components and circuits.